A peptidomimetic analogue of Pro-Leu-Gly-NH2 (PLG), compound 3, has been synthesized that contains a highly constrained spiro bicyclic type-II beta-turn mimic. Peptidomimetic 3 enhanced the binding of the dopamine receptor agonist ADTN to dopamine receptors by 40% at 10(-6) M. At this same concentration PLG enhanced the binding of ADTN by 26%. Like PLG, 3 exhibited a bell-shaped dose-response curve with the maximum effect occurring at a concentration of 10(-6) M. Because of the highly rigid nature of the spiro bicyclic type-II beta-turn constraint found in 3, these results lend strong support for the hypothesis that the biologically active conformation of PLG is a type-II beta-turn.